Carbon nanotubes are allotropes of carbon that is one of the most abundant elements on the earth. Carbon nanotubes are tubular structures in which carbon atoms are coupled to each other in a hexagonal honeycomb pattern and have an extremely small diameter in the nanometer range. Such carbon nanotubes exhibit characteristics of metals or semiconductors depending on their diameter and rolled-up shape. Carbon nanotubes are currently being investigated to overcome the limited mechanical and electrical properties of existing materials.
Particularly, single-walled carbon nanotube (SWCNT) bridges suspended between two electrodes or templates or three-dimensional networks thereof can find direct applications in a variety of electronic devices, including field emission displays (FEDs), nanotube interconnectors and nanosensors, due to their excellent electrical properties, such as high current density and ballistic conductance. In view of these advantages, various methods for forming single-walled carbon nanotube bridges and three-dimensional networks thereof have hitherto been proposed.
It is generally known in the art that carbon nanotube networks can be synthesized by selectively forming metal catalyst particles on desired sites of the surface of a silicon (Si) or silica (SiO2) substrate and growing carbon nanotubes in a two- or three-dimensional network on the metal catalyst particles.
Jung et al reported a method for forming single-walled carbon nanotube networks on a silicon or silica substrate patterned with nanoscale pillars by chemical vapor deposition (CVD) using methane as a carbon source (J. Phys. Chem. B 2003, 107, 6859-6864). However, there is a difficulty in directly applying the method to the fabrication of an electronic device because the silica substrate is made of non-conductive silica. A metal catalyst used is rendered inactive when the silicon substrate is used, leading to a marked decrease in the density of the networks. The vapor deposition technique, by which Fe or Co as the metal catalyst is deposited on the pillars to form thin films, involves two processing steps of inclining the patterned substrate right and left to deposit the metal catalyst on the upper and side surfaces of the nanoscale pillars, inevitably resulting in poor processing efficiency. Further, a high aspect ratio of the pillars makes it difficult for the catalyst to be uniformly deposited on the lower end portions of the pillars, thus leading to a low density of the carbon nanotubes.
U.S. Pat. No. 7,189,430 discloses a method for forming carbon nanotube networks without involving any additional catalyst deposition step. According to this method, oxide template structures are covered with a gold (Au) masking material and a mixture of xylene as a carbon source and ferrocene as a catalyst is directly used. However, the use of the masking material brings about a reduction in processing efficiency, and continuous supply of iron (Fe) present in the ferrocene increases the Fe concentration of the carbon nanotubes, eventually resulting in a decrease in the purity of the carbon nanotubes.
On the other hand, a technique is known in which catalyst particles are formed on nanoscale pillars by dipping and then carbon nanotube networks are formed using the catalyst particles. However, this technique has a problem in that the catalyst particles may aggregate or a large amount of the catalyst particles may be separated off from the pillars during subsequent cleaning, resulting in a decrease in the density of the carbon nanotube networks.